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Should We Worry About Interference in Emerging Fig. 1. Illustration of inter-satellite co-channel...

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  • Should We Worry About Interference in Emerging Dense NGSO Satellite Constellations?

    Christophe Braun, Andra M. Voicu, Ljiljana Simić and Petri Mähönen Institute for Networked Systems

    RWTH Aachen University Email: {cbr, avo, lsi, pma}@inets.rwth-aachen.de

    Abstract—Many satellite operators are currently planning to deploy non-geostationary-satellite orbit (NGSO) systems for broadband communication services in the Ku-, Ka-, and V-band, where some of them have already started launching. Conse- quently, new challenges are expected for inter-system satellite coexistence due to the increase in the interference level and the complexity of the interactions resulting from the heterogeneity of the constellations. This is especially relevant for the Ku- band, where the NGSO systems are most diverse and existing geostationary-satellite orbit (GSO) systems, which often support critical services, must be protected from interference. It is thus imperative to evaluate the impact of mutual inter-system interference, the efficiency of the basic interference mitigation techniques, and whether regulatory intervention is needed for these new systems. We conduct an extensive study of inter- satellite coexistence in the Ku-band, where we consider all recently proposed NGSO and some selected GSO systems. Our throughput degradation results suggest that existing spectrum regulation may be insufficient to ensure GSO protection from NGSO interference, especially due to the high transmit power of the low Earth orbit (LEO) Kepler satellites. This also results in strong interference towards other NGSO systems, where traditional interference mitigation techniques like look-aside may perform poorly. Specifically, look-aside can be beneficial for large constellations, but detrimental for small constellations. Further- more, we confirm that band-splitting among satellite operators significantly degrades throughput, also for the Ku-band. Our results overall show that the complexity of the inter-satellite interactions for new NGSO systems is too high to be managed via simple interference mitigation techniques. This means that more sophisticated engineering solutions, and potentially even more strict regulatory requirements, will be needed to ensure coexistence in emerging, dense NGSO deployments.

    Index Terms—satellite interference, NGSO coexistence, Ku-band

    I. INTRODUCTION

    With the ongoing demand for broadband services, network operators have been diversifying the range of deployed wire- less technologies and their applications. In this context, satel- lite communication systems are being increasingly used for e.g. backhaul infrastructure for on-board wireless connectivity in airplanes [1] and integration with cellular 5G to offer terrestrial broadband services [2]. Recently, a large number of satellite operators have applied to the US spectrum regulator FCC for permission to launch new non-geostationary-satellite orbit (NGSO) systems [3]. These systems target spectrum bands in the range of 10-52 GHz, i.e. the Ku-, Ka-, and V-

    band [4], and some have already been approved and started launching, e.g. SpaceX [5].

    NGSO satellite deployments are thus expected to undergo significant densification compared to existing systems. This will lead to challenging inter-satellite coexistence cases in shared bands due to (i) the increase in the interference level, and (ii) the expected high heterogeneity of the NGSO satellite systems, for which the current spectrum regulation is very permissive, as e.g. enforced by the FCC in the US. In this dynamic emerging satellite deployment landscape, it is impor- tant to understand the interference interactions among different NGSO systems and to what extent regulatory intervention may be needed to ensure NGSO-NGSO coexistence.

    The most challenging inter-satellite coexistence cases are expected in the Ku-band, due to the very heterogeneous sys- tems in terms of numbers of satellites (i.e. tens to thousands) and geometric orbit properties (e.g. circular, elliptical) that are set to operate in this band. Moreover, existing geostationary- satellite orbit (GSO) systems also operate in the Ku-band and must be protected from interference by NGSO systems, as enforced by regulation in the US [6], where applications for a license for these new systems have been initiated. Given this high heterogeneity and uncertainty about the exact parameters of emerging deployments, it is not yet clear whether traditional satellite interference mitigation techniques and existing regu- latory requirements are sufficient to ensure coexistence with the new NGSO systems.

    Although there is some prior work on the impact of interference and mitigation techniques for satellite systems, most authors considered only interference between GSO- NGSO systems, e.g. [7], [8]. Moreover, these works consid- ered NGSO legacy deployments with few satellites, where NGSO-NGSO interference was not an issue, as expected for emerging constellations. Importantly, NGSO-NGSO inter- system interference has been largely unaddressed in literature, with the notable exception of [9], [10]. The authors in [9] considered only two low Earth orbit (LEO) constellations and one medium Earth orbit (MEO) constellation, with a satellite diversity technique to mitigate interference. This is different to the emerging NGSO satellite deployments, where many more systems with different design parameters are expected to coexist. Consequently, it is not clear whether the interference mitigation technique analysed in [9] is efficient for dense deployments. The authors in [10] conducted an extensive

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  • Fig. 1. Illustration of inter-satellite co-channel interference among different systems in the downlink, showing the GSO system (green) and three NGSO systems (yellow, red, and blue). All ground stations are co-located on Earth and each of them communicates with a single available satellite from its corresponding constellation. For each given link, all other links (in different colors) are interfering links.

    study on the impact of NGSO-NGSO co-channel interference in terms of throughput for new NGSO constellations with interference mitigation techniques like look-aside and band- splitting in the Ka- and V-band. However, in these bands there are no GSO satellite systems and no NGSO systems with elliptical or geosynchronous orbits. By contrast, in the Ku-band, we expect such more challenging coexistence cases. Consequently, it is imperative to thoroughly analyse inter- system coexistence for satellite deployments in the Ku-band, due to the highly heterogeneous NGSO constellation proper- ties and NGSO-GSO interference interactions.

    In this paper we consider inter-satellite coexistence in the Ku-band, conducting an extensive study on the impact of both NGSO-NGSO and NGSO-GSO co-channel interference on throughput. We adopt the methodology in [10], which we extend to incorporate GSO systems and more diverse NGSO architectures, i.e. highly elliptical orbit (HEO) and geosynchronous constellations. We consider various traditional interference mitigation techniques like look-aside and band- splitting for several ground station locations in the US and Europe. Our work is thus the first comparative analysis on coexisting NGSO-NGSO and NGSO-GSO systems for a di- verse set of scenarios, enabling us to derive insights about the efficiency of interference mitigation techniques with respect to different orbits, transceiver parameters, and ground station locations on Earth.

    Our results indicate that current spectrum regulation may be insufficient to ensure efficient GSO protection from NGSO interference. Furthermore, the most promising traditional NGSO-NGSO interference mitigation technique is look-aside, however, its performance is highly sensitive to constellation properties and the locations of ground stations. This suggests that more sophisticated engineering solutions and potentially more strict regulatory constraints are required to ensure coex- istence of emerging NGSO deployments.

    The remainder of this paper is structured as follows. Sec- tion II presents the system model. Section III details the simu- lation setup. Section IV presents and discusses the throughput results. Section V concludes the paper.

    (a) Look-aside (b) GSO protection

    Fig. 2. Illustration of the satellite diversity interference-mitigation techniques (a) look-aside between NGSO systems (yellow and blue) and (b) GSO protec- tion between an NGSO system (blue) and a GSO system (red). Angles ϕ1 and ϕ3 occur when no interference mitigation is applied, whereas ϕ2 > 5◦ and ϕ4 > 30◦ occur with the two respective interference mitigation techniques.

    II. SYSTEM MODEL

    This section presents the system model used to study the impact of interference from NGSO satellite systems. We first elaborate the considered interference types and scenarios in Section II-A. We then present the considered interference mitigation techniques in Section II-B, our evaluation metric in Section II-C, and the satellite constellations in Section II-D.

    A. Interference Types & Scenarios

    We focus on co-channel co-polarized1 interference among NGSO-NGSO and among NGSO-GSO satellite systems op- erating in the Ku-band in the downlink. This is illustrated in Fig. 1 for different systems in terms of altitude, number of available satellites covering a given Earth location, and elevation angle of the selected satellite. There is one ground station per NGSO constellation and one ground station per GSO satellite, where all stations are co-located on Earth. We thus consider the worst-case interference where the directional antennas o

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